Field of Invention
The present invention relates to a luminescent material and a luminescent device and, in particular, to an organic electroluminescent material, an organic electroluminescent device and a quantum dot electroluminescent unit.
Related Art
With the advances in electronic technology, a light weight and high efficiency display device, for example an LCD (liquid crystal display) device, has been developed. However, the LCD device has some disadvantages, for example the viewing angle is not wide enough and the response time is not fast enough. Moreover, the LCD device requires polarizers and a backlight, thus increasing the power consumption, the weight and the cost.
Therefore, an organic electroluminescent display becomes the next generation flat display device due to its advantages of self-luminosity, no restriction on viewing angle, power conservation, simple manufacturing process, low cost, high response speed, full color and so on.
FIG. 1 is a schematic diagram of a conventional organic electroluminescent display. As shown in FIG. 1, the conventional organic electroluminescent device 1 includes an anode 11, a cathode 12, an organic electroluminescent layer 13 and a substrate 14. The cathode 12 can be disposed on the substrate 14, and the organic electroluminescent layer 13 includes a host material and a guest material. As to the organic electroluminescent device 1 emitting light, when a direct current is provided for the organic electroluminescent device 1, electron holes and electrons flow into the organic electroluminescent layer 13 respectively through the anode 11 and the cathode 12. Charge carriers move, meet, and then recombine in the organic electroluminescent layer 13 because of the potential difference caused by an applied electric field. The excitons generated by the recombination of the electrons and the electron holes may excite the host material to perform combination and then generate energy. The energy is then transferred to the guest material (phosphorescent material) thus generating light.
The organic electroluminescent device has a problem of insufficient color purity, the current technique utilizes a layer of quantum dot (QD) or a quantum dot complex composite material to produce a quantum dot organic electroluminescent device (QD-SOLED) to solve the problem of insufficient color purity. For example, the patent number CN101889480 discloses that a quantum dot is coated with a phosphorescent material (i.e. the above quantum dot complex composite material which is referred to as an electro-phosphorescent quantum dot in the patent application CN101889480; in order to be consistent with the term of the specification, the composite material formed by coating the quantum dot with the phosphorescent material is referred to as the electro-phosphorescent quantum dot hereafter), and then the electro-phosphorescent quantum dot is used as a guest material of an emissive layer thus to produce a quantum dot organic electroluminescent device (QD-OLED). The quantum dot organic electroluminescent device (QD-OLED) is referred to as the quantum dot electroluminescent device hereafter.
Although the quantum dot electroluminescent device may solve the problem of insufficient color purity, the host material must have fine electron and hole transport properties, and its triplet energy gap also needs to be higher than that of the guest material to avoid the energy lost caused by back energy transfer.
To make the phosphorescent material effectively work, selecting the host material is one of the key points of improving the efficiency of the components. The triplet energy gap of the selected host material must be higher than that of the guest material, so the energy may be effectively transferred. The host material itself requires semiconductor properties, i.e. having fine electron and hole transport properties. In addition, the host material also requires fine thermal stability, and then it may be possible to apply to the production line.
Currently, the organic electroluminescent devices produced by red and green guest materials mostly have fine lifespan and efficiency. However, the triplet energy gap of a blue guest material is relatively high, and the back energy transfer often results in relatively low luminous efficiency of the organic electroluminescent device. Therefore, it extremely requires a host material which meets the requirements of high triplet energy gap and sufficient thermal stability.